Components are commonly mounted onto a printed circuit board (PCB) using reflow soldering in conjunction with PCB's without through-holes. A preferred type of oven for reflow soldering is one which uses nonfocused infrared energy combined with natural convection to heat the board and the components, as well as the solder, without damaging the board or the components. A preferred method and apparatus are set forth in the following U.S. Pat. Nos. which are assigned to the assignee of the present application: 4,565,917; 4,654,502; and 4,833,301. Each of these patents describes a system and method in which infrared energy is generated by panels at a desired peak wavelength, or temperature, which can be accurately controlled. The hot panels also cause an exchange of heat through naturally occurring convection.
In the foregoing system, a conveyor transports the PCBs with the components and solder paste already placed thereon through a plurality of heating zones. Typically, each zone includes a heater panel both above and below the conveyor. Each zone is independent of the other zones, and the panels in each zone can be heated to a temperature independently of the other zones.
In reflow soldering, typically, a ramp-soak-spike heater panel recipe is used, in which the temperature of the panels in the first zone or zones (the ramp zone) is higher than the temperatures of the panels in at least one subsequent, intermediate zone or zones (soak zone), and in which the heater panel temperatures in the last zone or zones (the spike zone), are again increased above that of the soak zone. In the ramp zone, the solder flux is activated, and volatile flux components are driven off. The temperature of the heater panels in the soak zone or zones is lower than that in either the ramp or spike zones. The purpose of the "soak" zone is to allow larger components and areas in the center of the PCB which heat more slowly to reach the same temperature as the smaller components and edges of the PCB, respectively, which heat more rapidly. Also, cleaning of oxydized surfaces is promoted. In the spike zone, the temperature is increased rapidly to just above the melting temperature of the solder to produce the desired solder reflow. Thereafter, the PCBs are rapidly cooled to cause solidification of the solder.
In recent years, it has been determined that it is desirable to supplement the infrared and natural convection heating of the PCBs by using forced convection currents. Forced convection increases the heat exchange coefficient, and, therefore, increases the rate at which the PCBs and the components are heated, thereby increasing the throughput. In addition, forced convection can assist in reducing hot spots, and the shadowing effect occasionally noted with infrared panels. As a result, more uniform heating of the PCBs and the components is produced and more uniformly satisfactory results and better solder joints are achieved without damage due to overheating of small components.
A number of systems have been introduced to the market in which the infrared heating of the PCBs is supplemented with forced convection. However, in systems using forced convection, zone definition and accurate temperature control is oftentimes very difficult to maintain. Air flowing in one heated zone has a tendency to flow into or heat adjacent zones in an uncontrolled manner. Typically, air from hotter zones flows into cooler zones creating the potential of overheating the cooler zones. Also, heat transfer between zones occurs through conduction and radiation.
Because the middle or "soak" zone typically is sandwiched between two hotter zones, heat tends to be transferred to the soak zone from the surrounding warmer zones. Most ovens have no cooling capability, so that once a zone has become overheated, it is difficult to reduce the temperature to a desired level. Such uncontrolled heat exchange reduces the accuracy of the temperature control of each zone. This reduction in accuracy could impact the uniform PCB temperatures and, thus, the integrity of the solder joints could be affected or the PCB or components could be damaged.
Examples of prior art, convection-assisted systems are found in U.S. Pat. No. 4,909,430, U.S. Pat. No. 4,876,437, U.S. Pat. No. 4,938,410, and International Publication No. WO 91/04824. These, and other systems, typically recirculate the air through the use of a fan, drawing the air over heating coils or through a heated panel, and this heated air is then drawn over or through the PCBs on the conveyor. U.S. Pat. No. 4,938,410 shows a system in which fans are disposed both above and below the conveyor, and air is circulated past the PCBs in a generally horizontal direction, rather than vertically. Each of the foregoing apparatus recirculates heated air or some other gas exhausted from a zone or zones back to the intake side the zone or zones for reheating and further heating of the PCB's.
It is believed that each of these systems suffers from some degree of difficulty in controlling the temperature within each zone due to uncontrolled heat exchange between the zones, particularly as recirculation rates are increased.
Accordingly, it is an object of the present invention to provide better zone definition within a forced convection-assisted infrared solder reflow apparatus.
It is another object of the present invention to provide better control of the gas flow between zones in a forced convection-assisted infrared solder reflow apparatus.
It is another further object of the present invention to provide more precise temperature control of the gas within each zone of a forced convection assisted infrared solder reflow apparatus.